A senior Tory MP has asked the home secretary whether al-Qaeda sympathisers were mistakenly recruited by MI5.
Patrick Mercer, chairman of the Home Affairs counter-terror sub-committee, said sources told him six had sought to infiltrate the security service.
Four were ejected at the initial vetting stage but two got "further down the system", he told the BBC.
The Home Office declined to comment but Whitehall officials firmly rejected the claims, saying there was no evidence.
Mr Mercer said: "What concerns me is that not all of these individuals... have necessarily been nailed and that is why I've written to the home secretary to seek his reassurance."
Mr Mercer has written to Home Secretary Alan Johnson for clarification
The MP believes the security services rushed to try and take on Muslim recruits after the 7/7 bombings on London's transport network in 2007.
He told the BBC: "The trouble is when you start recruiting in a hurry... your enemies will try to take advantage of any gaps that have been created."
The former intelligence officer said three separate sources had told him of the claims, which came as no surprise to him.
He said: "Any subversive organisation worth its salt is trying to do to our intelligence agencies what we are trying to do to them, I don't think we should be surprised.
"I do think this is a very, very useful reminder that our enemies are out there, that they are alive and kicking and they wish to do us harm."
Mr Mercer has written to Home Secretary Alan Johnson for clarification on exactly how far, if at all, the reported al-Qaeda sympathisers got in the organisation before they were ejected.
Whitehall officials told BBC security correspondent Frank Gardner there was no evidence anyone with al-Qaeda sympathies had ever been recruited into MI5.
Two years ago the MI5 spoke openly about their drive to broaden recruitment among ethnic minorities.